1. Field of the Invention
This invention relates to coated hard metal tools suitable for use in cutting of metals, for example, turning, milling, threading, drilling, boring, etc., and a process for producing the same. The coated hard metal used herein comprises a substrate of cemented carbide consisting of WC, TiC, TaC, etc. and an iron group metal or a substrate of hard sintered alloy such as cermet consisting predominantly of TiC, TiN, etc., and a coating layer provided thereon consisting of a carbide, nitride, carboxide, carbonitride, oxide or solid solution thereof of Group IVa, Va or VIa element of Periodic Table, Al, Zr or the like, the coating layer having a higher hardness and wear resistance than the substrate.
2. Description of the Prior Art
In the field of cutting of metals, working conditions have become severer with the years and it has been required for cutting tools used to this end to increase the hardness, wear resistance and heat resistance. Cemented carbide tools are capable of satisfying these requirements to some extent, but tools of cemented carbides with various hard coating layers have lately spread. A typical form of the tools is shown in FIG. 1 in which a square insert is fixed to a holder. This is generally called a throwaway insert which is to be thrown away to make an exchange for a new insert after using eight cutting edge corners. In these coated cemented carbide tools, the surface of cemented carbide tip 4 is ordinarily coated by the CVD method, PVD method or the like. FIG. 1(b) is a cross-sectional view of insert 1 along line A--A and FIG. 2 and FIG. 3 are enlarged views of cutting edges 2 near dotted line B. As shown in these figures, the coated cemented carbide inserts of the prior art each have a thicker part near cutting edge 2 than other parts, which differs somewhat depending upon the shape of the cutting edge. When the coating film is of Al.sub.2 O.sub.3, in particular, that part is usually thicker. In general, if the coating film is thicker, the wear resistance is increased, but the toughness is lowered and chipping tends to occur. That is, the insert having a coating layer near the cutting edge as shown in FIG. 2 and FIG. 3 meets with a drawback that deterioration of the finished surface of a workpiece is caused by the disorder of wearing due to lowering of the toughness, breakage of the cutting edge and micro-chipping. Various proposals have been made so as to overcome this drawback.
For example, Japanese Patent Publication No. 37553/1973 describes a method of making a cemented carbide cutting tool comprising forming a coating layer consisting predominantly of TiC on the whole surface of a cemented carbide insert and removing by grinding the coating layer on the land between the cutting edge and groove for chip breaking to retain the coating layer on only the groove for chip breaking. However, this method has not been put to practical use yet, because it cannot be applied to inserts without chip breaker and inserts with bumpy chip breaker, and it encounters problems that the cutting edge is unfavourably affected by chip occurring by grinding and cutting edges 5' and 7' shown in FIG. 4 are brittle due to sharpness by grinding only the rake face. These problems cannot be solved by the similar method described in Japanese Patent Application No. 92732/1971.
By the prior art, therefore, the strength of an edge can be raised to some extent, but the wear resistance can only be held at most similar.